One of the recent commercially successful applications of Pressure Swing Adsorption (PSA) technology for bulk gas separation is direct production of 1-10 sl/m of ˜90% O2 from a compressed air stream for medical use by patients with Chronic Obstructive Pulmonary Disease (COPD) and other lung disorders. Rapid pressure swing adsorption (RPSA) processes employing total cycle times (tc) of less than ten seconds are generally employed for this application. Pelletized, N2 selective LiX or LILSX zeolite is often used as the preferred air separation adsorbent in these RPSA systems.
There are several commercial designs of RPSA Medical Oxygen Concentrators (MOC) employing various PSA process schemes for portable or stationary home use. Generally, the key design goals for a MOC are (a) reduction of the bed size factor [BSF, total amount of adsorbent in the unit/ton per day of oxygen production rate, (lbs/TPD O2)] which lowers adsorbent inventory resulting in a more compact and light weight unit, (b) enhancement of percentage O2 recovery R by the process [amount of O2 in product gas/amount of O2 in feed air per cycle×100(%)] in order to decrease the air compressor size and power which result in lighter unit and longer battery life (portable units), and (c) assembly of a compact, light-weight and easy to operate unit.
A classical four-step “Skarstrom PSA” cycle or some variation thereof is usually adapted for use in these RPSA schemes. The typical steps include (i) selective adsorption of N2 from compressed air by flowing air at a super-ambient adsorption pressure (PA) over a packed column of the zeolite to produce an O2 enriched effluent gas which is partly withdrawn as the product gas, (ii) counter-current depressurization of column to a near ambient final desorption pressure level (PD), (iii) counter-current back purge of the column at PD with a part of the O2 enriched product gas, and (iv) re-pressurization of column from PD to PA using either fresh compressed air (co-current) or a part of the O2 enriched product gas (counter-current) or both. The cycle is then repeated. The N2 enriched column effluent gases from steps (ii) and (iii) are wasted.
The conventional approach to reduce the BSF is to reduce the total cycle time (tc) of the RPSA system in order to increase the cyclic frequency of operation and hence, enhance the net rate of O2 production. A higher O2 recovery is generally obtained by preserving a portion of the air-like void gas in the column at the end of step (i) by using it to partially pressurize a companion column (pressure equalize) before step (ii) begins in order to reduce the loss of void gas O2 during step (ii). The amount of back purge gas is also minimized while maintaining the product gas purity in order to reduce BSF and increase O2 recovery.
At least two parallel adsorbent columns are typically needed in a PSA system to accommodate the pressure equalization step. At least two columns are also necessary to produce a continuous product stream so that when one column is undergoing step (i), the companion column carries out steps (ii)-(iv). However, synchronized control of operation of two columns in a rapid cycling situation tends to be difficult and subject to malfunction. A product buffer tank is often necessary for smoothing out product gas flow rate and composition.
Therefore, a need exists for a smaller, more efficient device to facilitate mobility and travel. The present invention fulfills this need among others.